Dia de Los Muertos is a time of remembering and communing with past relatives. Altars are erected to remember and celebrate those who have died.
In the Christian tradition it is called "All Saints Day." ("All Souls Day," Nov. 2, celebrates everyone who has died, not just the saints.) Following a passage in the Bible found in Hebrews, All Saints Day is a communion of the saints. When the Catholic traditions mixed with the Aztec traditions of Mexico, they merged and morphed into what is mostly recognized now as the celebration that we have today...a Day of the Dead/All Saints Day that is holy for all people.
In this celebration, altars are erected that feature "ofrendas" or offerings. Much like the carved pumpkins of Halloween (Hallowe'en...All Hallows Eve...the eve before All Saints Day...Hallow...to make holy...you get the picture), these ofrendas are offered to the dead to entice them to visit. They include things favored by the person when s/he was living.
And wonderfully decorated "calaveras" (skulls). These are often made from sugar, chocolate and amaretto.